WHITE : PETROGRAPHY OF THE BOSTON BASIN. 
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a rearrangement of the constituents to form a new series of secondary 
minerals. Particles of the unaltered glass sometimes remain; at 
other times it has only partly altered and hence gives a “ dusty ” 
appearance. The crystals which had time to assume their crystalline 
forms, in these rocks usually the quartz and feldspars, remain in this 
more or less altered ground mass. Professor Iddings (’89) has 
proposed the convenient term plienocrysts (or preferably plmnero- 
crysts) for the porpliyritic crystals. Finally came the type of 
rhyolites having the fluidal or lluxional arrangement of the ground 
mass (PL 1, Fig. 5). These erupted through the porpliyritic shell, 
from the molten interior, where they left behind the basic granite, 
which had slowly crystallized beneath and thus separated out, 
while the relatively acid residuum of the magma poured forth on 
the surface. This latter type, as indicated by the analyses, is the 
most acid of the series of rocks in this district, —that is to say, the 
percentage of silica is relatively highest; therefore, as the magma 
was not readily fusible, in flowing over the surface it quickly chilled, 
yielding few or no conspicuous plienocrysts, and being thus frozen 
as they flowed they maintained their flow lines and ropy character, 
forming a glass which later suffered devitrification. The devitrifica¬ 
tion in portions of the flow has given rise to a granophyric texture, 
like short dark-colored bristles arranged radially about the feldspar 
plienocrysts, in banks like axiolites or as rosettes. The thin sections 
often show these bands beautifully, flowing closely around the 
included plienocrysts as seen in PI. 1 , Fig. 5 ; PI. 2 , Fig. 6 ; and 
PI. 5 , Fig. 15. In various portions of the great volcanic ejectment 
of the Blue Hills we may trace almost every phase of the results of 
magma cooling under varying conditions, with their transition 
stages, 1 the variations illustrating very beautifully Professor Iddings’s 
idea of consanguinity in rock magmas. The various stages in this 
separation of the more acidic and the more basic portions of the 
magma constitute the various rock types described in this paper, 
starting with the more nearly neutral phases of the granites, which 
on gross analysis would probably run approximately from G5 % to 
70 % in silica, and passing thence to the most basic type, — the 
diorites, on the one hand, giving perhaps as low as 55 % of silica, 
and to the most acid type, the fluidal aporhyolites, on the other, 
1 For discussion of the causes of crystallization and relations of portions of a cooling 
magma see Iddings (’89 and ’92). 
For discussion of varieties of texture and descriptive terms applied to them see Bas- 
com (’96). 
